I thing the problem is the common ground of the boards in combination with grounding the input.

Chris

Try a "star" ground at the input connector of the phono preamp, where you take the turntable ground, and isolated metal "bath tubs" for both boards, also connected to the same star ground, that should solve it. You can use double-sided sticky tape to mechanically fix the bath tubs.

We added the input offset trim " just in case", to erode even the tiniest amount. A well build Paradise does not need the offset trim and cartridges can be swapped at will.
What happens here is typical. We have discussed this in detail before but for late comers going throw nearly 10,000 posts is hard.

I think we can safely request that people leave their boards powered up fora couple of days before taking any serious measurements from them, then we can eliminate the electrolytic capacitors from the mix. Does that make sense?

Many (DIY and professional) MC phono pre's are on the market, with DC coupled inputs, and this seems to be a non-issue. In fact, I asked that question over at John Curl's "Blowtorch" thread, and the experts commented that they never saw this as an issue (PMA you may remember as you were one of the experts replying to this).

Hi hesener,

yes I agree with your post. In 2008 I was asked to design a MC preamp. It was completely DC coupled. There was a small-lot production of that phono preamp, like 50 - 100 pcs. None of the preamps produced had problems with DC offset fluctuation, in other words all were stable and output DC offset voltage was like 1 - 2mV or less, depending on ICs used. The preamp was similar to my MM open project with linear 20dB stage added in front of the MM input terminals.

Try a "star" ground at the input connector of the phono preamp, where you take the turntable ground, and isolated metal "bath tubs" for both boards, also connected to the same star ground, that should solve it. You can use double-sided sticky tape to mechanically fix the bath tubs.

It´s more entertaining to hear perfect music with both psu as finding the right grounding with one, but i try it at the evening.

all were stable and output DC offset voltage was like 1 - 2mV or less, depending on ICs used.

Regards,

many thanks, thats exactly what we should expect here as well, the remaining offset being the normal offset for a IC opamp without nulling. the proposal from sq225917 makes perfect sense in that context.

I can increase the LF outputoffsetfluctuation by blowing to the pcb .
The offsetfluctuation is also higher with the 20kOhm on output than without .
I will check to support R15 by a Trimmerpoti when i get one .

I saw your circuit before, but the ground of both boards are common, only the ground of the turntable is decouples through DDRC, right? Chris

Yes you are still I would like you to try it (if posible) (it will/may help other people in the future). Having the DDRC in the player will (IMHO) harm nothing.

Also, the schema as shown makes sure that the -L and -R of the cartridge are separated (all the way) and that the player ground and signal-ground (the arm) are separated (you need one extra wire from the player to the RIAA).

After that all grounds are connected at a 'star'-point in the RIAA-box.